1. Field of the Invention
This invention relates to accelerometers, and more particularly to a unique mounting assembly which avoids problems of frequency interference.
2. Description of the Prior Art
Accelerometers are used in various applications to measure shock, vibration, acceleration or deceleration. Among the most popular types of accelerometers are piezoelectric accelerometers which comprise a mass acting upon a piezoelectric material. Accelerations move the mass to provide an electric indication from the piezoelectric material. Piezoelectric accelerometers are popular because they are very rugged and easily used. Often, these accelerometers include built-in amplifiers which increase their convenience. The mass in the piezoelectric accelerometers is usually undampened, but the accelerometers typically have a high natural frequency of greater than 30,000 Hertz (30 kHz), and usually greater than 40 kHz. Because the mass in the piezoelectric accelerometers is undampened, any high frequency vibration to which the accelerometer is subjected may be greatly amplified by the accelerometer.
In many applications, the accelerometer is mounted to the specimen to be monitored by means of a mounting block or other attaching device. For example, in U.S. Pat. No. 3,391,571, issued to Johanson, an accelerometer is attached to pavement by means of a tool which is driven into the pavement and has an attaching mechanism for the accelerometer. For another example, in pile capacity testing apparatus such as that shown in U.S. Pat. No. 3,960,008, the accelerometer is attached to a hammer which is dropped onto a concrete pile to determine the static load bearing capacity of the pile.
For either axial vibration or axial shock measurements, it is often necessary to mount the accelerometer parallel to the specimen to be monitored. This parallel mounting may be accomplished by using a mounting assembly comprising a block which is bolted or cemented to the specimen and has means for attaching the accelerometer to the block. A typical mounting block may be in the form of a small rectangular prism of approximately one to two inches in length which is formed of a rigid, secure, lightweight material.
A piezoelectric accelerometer may be operated up to frequencies close to its resonant point, which is usually in the neighborhood of 40 kHz. However, in order to obtain this high frequency response, the accelerometer needs a strong, rigid support with essentially direct contact between the accelerometer base and the surface of the specimen to which it is mounted. The accelerometer mounting block must not provide any cushioning or resilient effect which would isolate the accelerometer from the specimen and significantly reduce the high frequency response of the transducer. The mounting block must also be relatively lightweight to avoid attachment problems and the possibility of mass effects of the mounting block on the measured acceleration or vibration. For these reasons, aluminum has been used and has previously been considered the ideal material for the fabrication of accelerometer mounting blocks.
There is a major disadvantage or problem with aluminum mounting blocks. The natural frequency of the block is in the neighborhood of that of the accelerometer, i.e., about 40 kHz. Thus, incidental high frequency vibrations are amplified or reinforced by the combination of the mounting block and the accelerometer, causing quick saturation of the accelerometer amplifier or erratic signals which may make the lower level vibration or shock measurements difficult or impossible to read.